Hydraulic control circuit

ABSTRACT

A hydraulic control circuit having a control valve with a multiposition control spool having hydraulically set cutoff positions wherein the spool is hydraulically blocked against movement including a plurality of ports communicating with the control valve bore and suitable hydraulic control components for opening or closing lines connected to said ports. Further, a circuit for regenerative cylinder operation wherein the cylinder is positively blocked against movement in both directions when the control circuit is set for such condition and wherein the regenerative position of the control spool is hydraulically locked and a pressure relief valve is in circuit whereby, when the cylinder pressure exceeds a predetermined amount, the relief valve opens to release the hydraulic locking of the control spool whereby the control spool can move to a position to discontinue the regenerative flow of fluid to the cylinder.

United States Patent Christensen et al.'

HYDRAULIC CONTROL CIRCUIT Norman B. Christensen; Thomas A. Clark; RobertJ. Webers, all of Racine, Wis.

Assignee: Rex Chainbelt lnc.

Filed: Sept. 2, 1970 Appl. No.: 68,874

Inventors:

References Cited UNITED STATES PATENTS Schmiel et a1. ..l37/625.63 Rice..91/436 x 3,273,468 9/1966 Allen ..9l/436x [4 1 Feb.'22,19'72 PrimaryExaminerl'lenry T. Klinksiek Attorney-Hofgren, Wegner, Allen, Stellman8!. McCord [57] ABSTRACT A hydraulic control circuit having a controlvalve with a multiposition control spool having hydraulically set cutoffpositions wherein the spool is hydraulically blocked against movementincluding a plurality of ports communicating with the control valve boreand suitable hydraulic control components for opening or closing linesconnected to said ports. Further, a circuit for regenerative cylinderoperation wherein the cylinder is positively blocked against movement inboth directions when the control circuit is set for such condition andwherein the regenerative position of the control spool is hydraulicallylocked and a pressure relief valve is in circuit whereby, when thecylinder pressure exceeds a predetermined amount, the relief valve opensto release the hydraulic locking of the control spool whereby thecontrol spool can move to a position to discontinue the regenerativeflow of fluid to the cylinder.

12 Claims, 4 Drawing Figures HYDRAULIC CONTROL CIRCUIT BACKGROUND OF THEINVENTION This invention pertains to hydraulic control circuits havingvalve components providing for new and improved hydraulically controlledmultiposition operation of a control spool and which is positively heldin different positions and to such a circuit providing new and improvedregenerative cylinder operation with the cylinder positively blockedagainst movement in both directions when pressure fluid is not beingsupplied to either extend or retract the cylinder.

Circuits for regenerative cylinder operation are known wherein thecontrol spool has a position providing for regenerative operation of thecylinder; however, such a system is not positive in that pressurebalancing in opposite ends of the control spool is utilized, rather thana blocked column of fluid as in this application, to hydraulically lockthe spool in the regenerative position. Prior known units forregenerative cylinder operation have not provided for positive blockingof the cylinder against movement in either extended or retracteddirections, however, the circuit disclosed herein provides positiveblocking in both directions whereby inadvertent movement of the cylinderand mechanism connected to the piston rod thereof cannot occur.

SUMMARY An inventive concept embodied in the structure disclosedhereinrelates to a new and improved hydraulic control circuit for providingregenerative cylinder operation wherein the components can be set in acylinder blocking position and the cylinder is blocked against bothextending and retracting movements.

Another inventive concept embodied in the structure disclosed herein isthe utilization of a hydraulic control circuit wherein a control spoolhas plural positions and wherein the control spool has at least oneintermediate position where it is hydraulically locked by a blockedcolumn of fluid to prevent movement thereof until said blocked column offluid is free to flow to drain.

An object of the invention is to provide a new and improved hydrauliccontrol circuit providing for regenerative cylinder operation andembodying the concepts set forth above which provides for reliable, safeoperation of a cylinder with blocking of movement of the cylinderagainst movement in either direction when the system is inactive and,further, which provides for regenerative flow of fluid from the rod endof the cylinder to the other end of the cylinder until such time as apredetermined cylinder pressure is exceeded and the regenerative featureis discontinued.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of aregenerative manifold embodying the hydraulic control circuit utilizingUSASI Symbols and with the control valve shown in vertical section andwith the parts positioned in neutral position for blocking of movementof the cylinder in both directions;

FIG. 2 is a view, similar to FIG. 1, showing the components of thehydraulic control circuit positioned for extension of the piston rod ofthe cylinder in the regenerative cycle of operation;

FIG. 3 is a view similar to FIG. 2 except for showing the change inposition of certain parts to have the piston rod extending and acting ona load and under full system pressure; and

FIG. 4 is a view similar to FIG. 1 and showing the components of thehydraulic control circuit positioned for retracting the piston rod ofthe cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to FIG. I,the hydraulic control circuit, which can be constructed as a manifolddevice, has a series of major components including the main controlvalve having a bore 11 which movably receives a control spool 12. Thecontrol valve 10 has a pair of cylinder lines 15 and 16 extending fromvalve ports 17 and 18, respectively, and connectable to a cylinder 20having a piston 21 with a piston rod 22. The line 15 connects to the rodend of the cylinder, while the line 16 connects to the opposite end ofthe cylinder.

Reference is made herein to a number of lines" which can be in the formof either internal body passages or external piping, both variationsbeing well known in the art.

Other major components are a four-way directional control valve,indicated generally at 25, having solenoids 26 and 27 providing forshiftthereof from either side of a spring-centered position. Apilot-controlled, four-way valve, indicated generally at 30, isresponsive to hydraulic pilot action at either ends thereof under thecontrol of the position of the directional control valve 25 and is theprimary control of connection of the main fluid power lines for thecylinder to either pressure or tank.

A three-position pilot valve, indicated generally at 31, has aconnection to a line 32 connecting to a port 33 at one end of the bore11 of the control valve 10. This three-position pilot valve isspring-centered and hydraulically piloted at both ends to control theconnection of the port 33 either to drain or to pressure from either oftwo lines to be described subsequently. Two additional primarycomponents are a settable pressure relief valve, indicated generally at35, and a pressure-responsive valve, indicated generally at 36.

The pressure fluid is supplied to the circuit through a pressure line40. The circuit has a tank line 41 and a drain line 42. For operation ofthe cylinder 20 in either extending or retracting movement, fluid underpressure ultimately reaches one or the other of a pair of main lines 45and 46 which are connectable to the control valve and communicate withthe bore thereof through the ports 47 and 48, respectively. In thecylinder-blocked position of FIG. I, it will be noted that there is nofluid communication between the main line ports 47 and 48 and thecylinder line ports 17 and 18. This blocking is accomplished by a pairof intermediate lands 50 and 5] on the control spool. Additionally, thecontrol spool has the control lands 52 and 53 at the ends thereof whichare responsive to fluid pressure conditions at opposite ends of thevalve bore to determine which of several positions the control spoolwill be The control spool 12 is in its right-hand limit position in FIG.1 with said position being maintained by a compression spring 55 in anend of the valve bore acting between a wall of the control valve casingand extending into a recess in the control land 52 of the control spool.There is no fluid pressure being exerted against either end of thecontrol land at this time. The control land 53 of the control spoolthrough port 33 and the three-position valve 31 is exposed to drainthrough a line 56. The control land 52 of the control spool is in fluidcommunication with first and second fluid ports 60 and 61 in the controlvalve body. These ports are spaced lengthwise along the length of thevalve bore with the port 61 being at the end of the bore and the firstport 60 being intermediate thereof and the opposite end of the bore. Theport 60, through a line 62, is at all times connected to the drain line63 which is an extension of the drain line 56. The second fluid port 61can be connected to pressure through passage means, to be described;however, in the positioning of the parts in FIG. I the passage means isnot connected to pressure through the directional control valve 25. Theother connections to the port 61 are for the purpose of selectivelyconnecting this port to the drain line 42 and, therefore, there is nopressure acting on the control land 52 with the positioning of the partsshown in FIG. I.

From the locked position of FIG. 1, the circuit can be shifted to causeextension of the cylinder rod or retraction thereof. For purposes ofdescription, it will be assumed that the desired operation is to causeextension of the rod which, with little load on the rod, will causeregenerative operation and the positioning of the parts as shown in FIG.2. To place the circuit in the condition shown in FIG. 2, the solenoid27 of the directional control valve 25 is energized to shift this valveto the position shown in FIG. 2 wherein a line 65 connected to thepressure line 40 directs pressure fluid through the directional controlvalve 25 to a control line 66 having a branch line 67 connected to thepilot 68 of the pilot-controlled four-way valve 30. Line 66 also extendsto passage means having a first branch 69 and a second branch 70. Theactuation of the pilot section 68 of the valve 30 shifts this valve tothe position shown in FIG. 2 wherein pressure fluid from line 40 passesthrough this valve and connects to the main powerline 46 with the otherpowerline 45 being connected to the tank line 41 through thepilot-controlled fourway valve 30. Thus, shifting of thepilot-controlled four-way valve 30 conditions the lines 45 and 46 tohave one connected to pressure and the other connected to tank wherebyconnections can be made to the cylinder dependent upon the position ofthe control spool 12 of the control valve 10.

The positioning of the control spool 12 is accomplished by a pressuredifferential acting on both ends of the control spool through thepassage means having the first branch 69 and the second branch 70. Thesecond branch passage 70 extends to the three-position pilot valve 31,with pilot pressure acting through a line 71 to shift the valve 31 tothe position shown in FIG. 2 and connect a passage continuation 72 ofthe second branch passage 70 to the line 32 leading to the port 33 ofthe control valve whereby system pressure is exerted against the controlland 53 to act against the urging of the spring 55. The first branchpassage 69 has an orifice 74 which causes a pressure drop downstreamthereof as fluid flows to the port 61 and which can flow into the valvebore and out through the port 60 to the drain line 42 through the line63. This flow condition occurs initially upon actuation of the solenoid27, as clearly shown by the initial position of the control spool atsuch time in FIG. 1. This condition terminates when the control spool,moved by a pressure differential, reaches the position shown in FIG. 2where the control land 52 has moved to the position to block the firstport 60. This movement has occurred because of the orifice 74 causing apressure drop whereby the pressure, differential is sufficient toovercome the force of the spring 55 and have the greater pressure actingon the control land 53 shift the control spool toward the left as viewedin FIG. 2.

Shifting the control spool to the position of FIG. 2 positions theintermediate spool lands 50 and 51 whereby the main line 46 connected tothe pressure line 40 communicates with the cylinder line 16 through theports 18 and 48. The control valve ports 17 and 47 are still blockedfrom communication by coaction of the intermediate spool land 50 with araised land of the valve bore. This results in supplying pressure fluidto the cylinder to extend the rod, with flow from the cylinder throughthe cylinder line entering the bore of the valve through the port 17.This flow does not go to tank through main line 45 but is used in theregenerative cycle to add to the fluid delivered through line 16 to thecylinder. This regenerative effect is obtained by an internal passage 80in the control spool 12 having passages 81 and 82 communicating with thebore whereby fluid delivered from the rod end can enter passage 81, pasthrough the passage 80 and out the passage 82 for delivery to the port18 and to the cylinder line 16.

In reaching the position of FIG. 2, the control spool has travelleduntil it reached a hydraulically set cutoff position where flow from theleft-hand end of the valve bore was blocked by blocking the port 60. Thecontrol spool shifts from the position of FIG. 2 to the position of FIG.3 to discontinue the regenerative cycle only in response to the pressureof the fluid acting to extend the cylinder. The operation is obtained bycontrol obtained from the pressure relief valve 35 which is connected bya line 83 to the second port 61 of the control valve. A line 84 connectsthe outlet of the pressure relief valve to the drain line 42. Underload, the pressure in line 46 builds up, which pressure buildup istransmitted to the control line 66 connecting to the branch passages 69and 70 through the pilot-controlled four-way valve 30 whichcross-connects lines 46 and 66 through the line 65. This buildup inpressure in branch passages 69 and 70 does not result in any shift ofthe control spool 12 until the pressure exceeds the setting of therelief valve 35. When the pressure is exceeded, the valve shifts andflow to drain from port 61 through lines 83 and 84 releases the blockedfluid from the end of the bore whereby the greater pressure acting onthe control land 53 shifts the control spool to the position shown inFIG. 3. This shifted position of the pressure relief valve 35 is shownin FIG. 3.

As a result of the shift of the control spool 12 to its left-hand limitposition, shown in FIG. 3, the intermediate land 50 of the valve hasmoved to a position to no longer block communication between the ports47 and 17 whereby the cylinder line 15 can communicate with the mainline 45 which, through the pilot-controlled, four-way valve 30, connectsto the tank line 41. Thus full system pressure is available to extendthe cylinder rod acting under load. The position of the control spool12, as shown in FIG. 3, at the left-hand end of the bore is thedirectional control position of the control spool in the sense that thecontrol spool 12 in this position permits direct flow through to thecylinder lines 15 and 16 and the direction of pressure application tothe cylinder is solely dependent upon which of the lines 45 and 46 areconnected either to tank or pressure. This is illustrated by the sameposition of the control spool 12 shown in FIG. 4, which Figureillustrates the positioning of the circuit components when the cylinderis retracting and which will be described more fully subsequently.

At the end of the desired extension of the cylinder, the solenoid 27 isdeenergized with the result that the directional control valve 25centers, which connects control line 66 to tank whereby branch passages69 and 70 are connected to drain. This permits centering of thethree-position valve 31 which places the port 33 in communication withdrain and also connects the opposite end of the valve bore with drain tohave the spring 55 return the control spool 12 to the position of FIG. Iblocking the cylinder.

The retraction of the cylinder is accomplished by positiorr ing of thecircuit components, as shown in FIG. 4, resulting from energization ofthe solenoid 26 of the directional control valve. This shifts the lattervalve to the position shown in FIG. 4 whereby a pressure signal througha signal line acts on the pilot section 101 of the valve 30 to shift thevalve to the position shown whereby pressure from line 40 communicateswith main line 45 and main line 46 is connected to the tank line 41. Thecontrol spool 12 is initially in the position shown in FIG. 1 whichblocks communication between the main lines 45 and 46 and the cylinderlines 15 and 16. The main line 45 is now the pressure line and itconnects to control lines 105 and 106, branching off therefrom, with thecontrol line 106 having a pilot line 107 to the three-position pilotvalve 31 to shift the valve to the position shown in FIG. 4 which placesthe valve port 33 in communication with the branch control passage 106to exert pressure on the control land 53 and urge the control spooltoward the left. The movement of the control spool to the left ispermitted, to the regenerative control position, by outflow through theport 60, but the spool must travel to the left limit position and,therefore, there must also be flow out through the port 61. This isaccomplished by opening of the pressure-responsive valve 36 which isshiftable against the urging of a spring by a pilot line 110, branchingfrom the control branch passage 105, to shift the pressure-responsivevalve 36 to the open position shown in FIG. 4. This connects the port 61of the control valve 10 to the drain line 42 through a line 111connected to the outlet side of the pressure-responsive valve. As aresult, the control spool 12 can shift to the left limit position, whichthen places the main lines 45 and 46 in communication with the cylinderlines 15 and 16. Upon deenergization of the solenoid 26, the same actionoccurs as upon deenergization of the solenoid 27, with release of allpressure on the control spool 12, whereby the spring 55 can return it tothe cylinder-blocking position shown in FIG. 1.

In order to prevent any inadvertent operation and shift of thethree-position pilot valve 31, the branch control line 105 extends tothe drain line 42 and has a restricted flow communication therewiththrough an orifice 115 whereby'there cannot be any buildup in pressurein the control passages 105 and 106 which might cause shift of the valve31 from the neutral position thereof when the cylinder is blocked, asshown in FIG. 1. Similarly in the blocked condition of PEG. 1, the line66, which communicates with the branch passages 69 and 70, is connectedthrough the directional control valve to the drain line 42 so that therecannot be any inadvertent buildup of pressure acting on thethree-position valve 31 tending to shift it to the position of FIG. 2.

From the foregoing, it will be seen that a hydraulic control circuitproviding for regenerative cylinder operation has been provided whereinthe cylinder, when the circuit is in neutral position, is positivelyblocked against movement in either direction and further wherein thecontrol spool disclosed herein is positively-hydraulically locked inposition and by a system which provides for plural positioning of acontrol valve spool by selective control of outflow from portsassociated therewith.

We claim:

1. A hydraulic control circuit including a control valve having a borewith a movable control spool therein, a pair of control lands adjacentopposite ends of said spool, a first control fluid port to said bore forcommunicating fluid with said one control land to urge said controlspool in one direction, yieldable means urging said control spool in theopposite direction, a plurality of fluid ports to said bore spaced apartlengthwise thereof for coaction with the other of said control lands, aseparate control line to each of said last-mentioned ports, and means inat least one of said control lines to alternatively open or block therespective control line whereby the position of said control spool underthe urging of fluid in said first control port is controlled by the openor closed condition of said control lines.

2. A hydraulic control circuit including a control valve having a bore,a movable control spool having a plurality of hydraulically set controlpositions at different locations along the length of said bore, aplurality of control lands on said spool with one control land adjacenteach end thereof, a first control port to said bore for selectivelydirecting fluid under pressure against one of said control lands, secondand third control ports to said bore adjacent an end thereof remote fromsaid first control port and spaced apart lengthwise along said bore, apair of control lines connected one to each of said second and thirdcontrol ports, a shutoff valve in the control line connected to saidsecond port which is nearest said end of the bore whereby said controlspool can move a full stroke when said shutoff valve is open and canmove a lesser stroke to an intermediate locked position determined bythe location of the third port and which when blocked by said othercontrol land terminates fluid outflow from the bore.

'3. A hydraulic control circuit as defined in claim 2 wherein saidsecond port is connected to said fluid under pressure through anadditional line having an orifice therein to provide a pressure drop,and spring means in said bore urging said control spool toward saidfirst port.

4. A hydraulic circuit for regenerative cylinder operation including apair of cylinder lines connectable to a cylinder, a control valveconnected to said lines and having a control spool, a pair of inputfluid lines to said control valve individually connectable to pressureor tank, said control spool having three positions wherein one end limitposition places said cylinder lines in fluid communication with saidinput fluid lines for operating a cylinder in a selected direction,another limit position of the control spool blocks communication betweensaid lines for locking a cylinder, and an intermediate locking meansincludes a control fluid line having a pressureresponsrve valve wherebya predetermined pressure in the control fluid line will open saidpressure-responsive valve to release the hydraulic lock on the controlspool.

7. A circuit as defined in claim 4 including hydraulic locking means tohold said control spool in the intermediate position, said meanscomprising a passage with branches communicable with opposite ends ofthe control spool and an orifice in a first of said branches to obtain arelatively reduced pressure therebeyond, a bore for said control spool,an outlet from said bore near one end thereof and communicable with saidfirst branch line whereby pressure applied through said branches causesmovement of the control spool toward one end because of the pressuredifferential, said movement stopping and the control spool beinghydraulically locked when said bore outlet is closed by the controlspool.

8. A circuit as defined in claim 7 wherein a'drain line connects withsaid first branch passage, a pressure relief valve in said drain lineopenable at a predetermined pressure to drain said first branch passagedownstream of said orifice and permit movement of said control spool tosaid one end limit position.

9. A circuit as defined in claim 8 wherein said control spool hasinternal passage means to provide said cross-connection between thecylinder lines.

10. A circuit as defined in claim 8 including a second pres'sure-responsive valve openable by pressure to drain said first branchpassage and said one end of the bore and permit full movement of thecontrol spool to said one end limit position whereby said control spoolcan move to said one end limit position either at the termination ofregenerative operation or when a cylinder is to operate in retractingmovement.

11. A hydraulic circuit for regenerative cylinder operation comprising apair of cylinder lines connectable to the cylinder, a control valveconnected to said lines and having a bore with a control spool, a pairof fluid lines to said control valve individually connectable topressure or tank, a three-position pilot valve having an outletconnected to one end of said bore whereby in one position the pilotvalve connects the bore to drain and in the other two position connectssaid bore to pressure fluid to move said control spool, a pair of portsadjacent the opposite end of the bore with a first port spacedlengthwise of the bore from the second of the ports, said second portbeing connected by passage means to said three-position pilot valve andto a fluid pressure line with an orifice in said passage upstream ofsaid second port whereby a pressure differential applied to said controlspool causes movement thereof toward said second port, said first portbeing connected to drain to permit fiow from the second port to thefirst port, said movement of the control spool being stopped andhydraulically locked when the control spool blocks said first port, andtwo alternate systems for bleeding fluid from said second port to permitmovement of the control spool to an end limit position.

12. A circuit as defined in claim 11 wherein one of said two alternatesystems includes a drain line connected to said second port with apressure relief valve openable in response to a predetermined pressureapplied to the cylinder, and the other system includes apressure-responsive valve connected to said second port and responsiveto pressure applied to one end of said three-position pilot valve.

1. A hydraulic control circuit including a control valve having a borewith a movable control spool therein, a pair of control lands adjacentopposite ends of said spool, a first control fluid port to said bore forcommunicating fluid with said one control land to urge said controlspool in one direction, yieldable means urging said control spool in theopposite direction, a plurality of fluid ports to said bore sPaced apartlengthwise thereof for coaction with the other of said control lands, aseparate control line to each of said last-mentioned ports, and means inat least one of said control lines to alternatively open or block therespective control line whereby the position of said control spool underthe urging of fluid in said first control port is controlled by the openor closed condition of said control lines.
 2. A hydraulic controlcircuit including a control valve having a bore, a movable control spoolhaving a plurality of hydraulically set control positions at differentlocations along the length of said bore, a plurality of control lands onsaid spool with one control land adjacent each end thereof, a firstcontrol port to said bore for selectively directing fluid under pressureagainst one of said control lands, second and third control ports tosaid bore adjacent an end thereof remote from said first control portand spaced apart lengthwise along said bore, a pair of control linesconnected one to each of said second and third control ports, a shutoffvalve in the control line connected to said second port which is nearestsaid end of the bore whereby said control spool can move a full strokewhen said shutoff valve is open and can move a lesser stroke to anintermediate locked position determined by the location of the thirdport and which when blocked by said other control land terminates fluidoutflow from the bore.
 3. A hydraulic control circuit as defined inclaim 2 wherein said second port is connected to said fluid underpressure through an additional line having an orifice therein to providea pressure drop, and spring means in said bore urging said control spooltoward said first port.
 4. A hydraulic circuit for regenerative cylinderoperation including a pair of cylinder lines connectable to a cylinder,a control valve connected to said lines and having a control spool, apair of input fluid lines to said control valve individually connectableto pressure or tank, said control spool having three positions whereinone end limit position places said cylinder lines in fluid communicationwith said input fluid lines for operating a cylinder in a selecteddirection, another limit position of the control spool blockscommunication between said lines for locking a cylinder, and anintermediate control spool position which provides regenerativeoperation for a cylinder by cross-connecting said cylinder lines.
 5. Acircuit as defined in claim 4 including means hydraulically locking saidcontrol spool in the intermediate position.
 6. A circuit as defined inclaim 5 wherein the hydraulic locking means includes a control fluidline having a pressure-responsive valve whereby a predetermined pressurein the control fluid line will open said pressure-responsive valve torelease the hydraulic lock on the control spool.
 7. A circuit as definedin claim 4 including hydraulic locking means to hold said control spoolin the intermediate position, said means comprising a passage withbranches communicable with opposite ends of the control spool and anorifice in a first of said branches to obtain a relatively reducedpressure therebeyond, a bore for said control spool, an outlet from saidbore near one end thereof and communicable with said first branch linewhereby pressure applied through said branches causes movement of thecontrol spool toward one end because of the pressure differential, saidmovement stopping and the control spool being hydraulically locked whensaid bore outlet is closed by the control spool.
 8. A circuit as definedin claim 7 wherein a drain line connects with said first branch passage,a pressure relief valve in said drain line openable at a predeterminedpressure to drain said first branch passage downstream of said orificeand permit movement of said control spool to said one end limitposition.
 9. A circuit as defined in claim 8 wherein said control spoolhas internal passage means to provide said cross-connection between thEcylinder lines.
 10. A circuit as defined in claim 8 including a secondpressure-responsive valve openable by pressure to drain said firstbranch passage and said one end of the bore and permit full movement ofthe control spool to said one end limit position whereby said controlspool can move to said one end limit position either at the terminationof regenerative operation or when a cylinder is to operate in retractingmovement.
 11. A hydraulic circuit for regenerative cylinder operationcomprising a pair of cylinder lines connectable to the cylinder, acontrol valve connected to said lines and having a bore with a controlspool, a pair of fluid lines to said control valve individuallyconnectable to pressure or tank, a three-position pilot valve having anoutlet connected to one end of said bore whereby in one position thepilot valve connects the bore to drain and in the other two positionconnects said bore to pressure fluid to move said control spool, a pairof ports adjacent the opposite end of the bore with a first port spacedlengthwise of the bore from the second of the ports, said second portbeing connected by passage means to said three-position pilot valve andto a fluid pressure line with an orifice in said passage upstream ofsaid second port whereby a pressure differential applied to said controlspool causes movement thereof toward said second port, said first portbeing connected to drain to permit flow from the second port to thefirst port, said movement of the control spool being stopped andhydraulically locked when the control spool blocks said first port, andtwo alternate systems for bleeding fluid from said second port to permitmovement of the control spool to an end limit position.
 12. A circuit asdefined in claim 11 wherein one of said two alternate systems includes adrain line connected to said second port with a pressure relief valveopenable in response to a predetermined pressure applied to thecylinder, and the other system includes a pressure-responsive valveconnected to said second port and responsive to pressure applied to oneend of said three-position pilot valve.